Here is the item again:
To make his long shift at Bernie's Burger Emporium
tolerable, (A)
Kelvin used ketchup, (B)
to make smiley faces on the burgers
(C) that he assembled
at the grill.
You correctly realized that the comma after ketchup is wrong. To make smiley faces on the burgers ... is an infinitive phrase. When you attach an infinitive phrase to the end of a main clause, you need no punctuation.
To fix the problem, you chose to do this:
To make his long shift at Bernie's Burger Emporium bearable, Kelvin used
ketchup to make smiley faces on the burgers
that he assembled at the grill.
You have correctly employed Punctuation Rule 5:
Main clause + Ø + infinitive phrase.
What a brain you have!